The document discusses three types of relationships between tables in a database: one-to-one, one-to-many, and many-to-many. In a one-to-one relationship, each record in one table is linked to only one record in another table. A one-to-many relationship occurs when each record in one table links to only one record in another table, but records in the second table can link to multiple records in the first. A many-to-many relationship exists when records in one table can link to multiple records in another table and vice versa; this requires a junction table.

2. Relationships
Tables in a database can be linked to each other.
The relationship between linked tables can be described in one of three
different ways:
 one-to-one (1:1)
 one-to-many (1:m)
 many-to-many (m:n)

3. One-to-one (1:1)
Two tables have a one-to-one relationship when:
every record in the first table maps to one record in the second table

4. One-to-one (1:1)
A pupil has only one Young Scot Card, and each Young Scot Card is registered
to only one pupil
Pupil
Young Scot Card

5. One-to-one (1:1)
PupilPupil Young Scot
Card
Young Scot
Card
has

6. One-to-many (1:m)
Two tables have a one-to-many relationship when:
each record in the first table maps to only one record in the second table
and
each record in the second table maps to multiple records in the first table

7. One-to-many (1:m)
Pupil
School
A pupil can be enrolled in only one school, but a school can enrol many pupils

8. One-to-many (1:m)
PupilPupil SchoolSchool
enrolled in

9. Many-to-many (m:n)
Two tables have a many-to-many relationship when:
each record in the first table maps to multiple records in the second table
and
each record in the second table maps to multiple records in the first table

10. Many-to-many (m:n)
Pupil
Course
A pupil can study many courses and a course can have many pupils studying it

11. Many-to-many (m:n)
PupilPupil CourseCourse
studies

12. Many-to-many (m:n)
It is not possible to represent a many-to-many relationship between 2 tables in
a database
Instead we need to have a junction table (or join table) in the database

13. Many-to-many (m:n)
Pupil
Course
Course taken becomes a join table
Course Taken